Sens. Barack Obama and Hillary Rodham Clinton separately declared their support Tuesday for a March 31, 2008 cutoff in funds for the Iraq war, two Democratic presidential front-runners abruptly shifting positions on an issue at the heart of the 2008 race.

At first, legislation to raise the minimum wage loomed as a clean, quick triumph for Democrats eager to celebrate their new majority in Congress. Two months later, it stands as an early lesson in the limits of their power.